1. Field of the Invention
The invention relates to an expansion valve and to a method for controlling it, in particular in the form of vehicle air-conditioning systems which are operated with CO2 as refrigerant and have a valve housing with a feed opening and a discharge opening, and with a valve member which can be displaced out of a valve seat of a through-flow opening arranged between the feed opening and discharge opening to allow the refrigerant to flow through.
2. Description of Related Art
Carbon dioxide (CO2) is the preferred refrigerant for refrigerant cycles of air-conditioning systems of motor vehicles of the future, since this substance is very safe in the event of accidents, on account of being non-combustible, and furthermore it is not considered an environmental pollutant. CO2 refrigeration cycles, unlike R134a refrigeration cycles, are also operated in the supercritical range.
An expansion valve which is used in refrigerant cycles of air-conditioning systems using CO2 is known from DE 100 12 714 A1. This expansion valve has a throttling opening with a fixed cross section for transferring the refrigeration medium from the high-pressure side to the low-pressure side for the purpose of pressure expansion. This cross section is always open for through-flow. If an excess pressure is formed on the high-pressure side in the refrigerant cycle, a bypass valve connected in parallel with the throttling opening is opened, so that the excess pressure which exceeds the optimum high pressure is reduced. The bypass valve only opens when a predetermined threshold value is exceeded on the high-pressure side.
This arrangement represents a functionally reliable configuration of an expansion valve, but it is necessary for both the setting of the threshold value and of the orifice diameter to be matched to the particular air-conditioning system, in order to achieve a maximum performance coefficient over the entire range of applications of the air-conditioning system.
DE 102 19 667 A1 has disclosed an expansion valve with an electronic control which has an electrically actuable device for displacement of a valve member, with a further throttling location, the passage cross section of which can be adjusted in a manner coupled to the passage cross section of the first throttling location, being provided arranged in series with the first throttling location. This series connection of at least two throttling locations, of which at least one can be actuated by a solenoid valve, means that the pressure difference at each individual throttling location is lower than if there is just one throttling location. This increases the accuracy of control. In particular, it is possible to thereby deal with the discrepancies in the pressure difference which occur between summer and winter.
However, this solution has the drawback of requiring a complex structure. Actuation of the solenoid valve requires the use of a pressure and temperature sensor or a control box with software in the control circuit, making this expansion valve expensive to produce and assemble.